Dies are used for the extrusion of various types of materials and in particular for forming an extrudate into a sleeve or tubular form. Such dies are used with various types of thermoplastic materials and elastomeric materials. The molten resin is forced through an annular orifice which forms the final tubular extrudate. Normally the molten resin enters the die chamber in one or more streams of material caused by the passing of the molten resin past the radially extending legs of a spider before being rejoined in the extrusion die chamber. The locations at which the streams of resin rejoin have been found to form a seam or reduced gauge in the final tubular extrudate which, depending upon the particular product being formed from the extrudate, can present a weakened area. Furthermore, the rejoining of the streams of material may result in the formation of air pockets which produce blisters on the tubular extrudate after passing through the extrusion die.
This problem has been recognized in the art, particularly in the extrusion of thermoplastic materials. U.S. Pat. No. 4,509,907 attempts to solve this problem by the use of an extrusion head which forces the streams of thermoplastic material through flow passages having tapered cross-sectional configurations in order to develop a back pressure within the die chamber to assist in blending the separate streams of resin.
U.S. Pat. No. 3,146,495 discloses a molding machine which attempts to overcome this problem by passing the separated streams of material caused by the spider legs through a series of skewed flow passages formed in a baffle plate to create a swirling effect whereby the individual flow streams are more easily rejoined into a final mass before passing through the extrusion die.
U.S. Pat. No. 3,266,092 discloses a flow die which attempts to solve this problem by the formation of a die anchor ring having four inner overlapping arcuate flow passages and four outer overlapping flow passages, all of which have tapered ends which nest within each other. With this configuration the molten material is divided into four inner streams and four concentric outer streams which are caused to overlap in order to form a pair of concentric sleeves which are then merged together in forming the final tubular extrudate.
Still other prior art extrusion dies and equipment have been developed in an attempt to eliminate this problem or other problems similar thereto. Examples of these prior art devices are shown in U.S. Pat. Nos. 3,709,645 and 4,165,210 and in German Pat. No. 1,940,195.
Although these known prior art devices may provide solutions to the problems for which they were developed, they are used primarily for thermoplastic materials which are forced through the die at a considerably reduced pressure than the pressures exerted on a mass of molten elastomeric material, such as that used in the formation of an elastomeric tubular extrudate used in the formation of inner tubes for pneumatic tires. For example, the extremely high pressures that would be exerted against the slightly tapered surfaces of the die anchor ring such as shown in U.S. Pat. No. 3,266,092, would require an extremely thick and heavy ring in order to eliminate outward bowing of the end wall area of the ring and adjacent metal components.
Therefore, the need has existed for an improved restriction insert for an extrusion die, which die is intended primarily for use in the extrusion of an elastomeric tubular extrudate to reduce or eliminate the uneven gauge and trapping of air in the extrudate which results when the molten elastomeric mass flows around the spider legs, and whereon the insert is able to withstand the extremely high pressures which are exerted on the restriction insert.